A victim is helped from the rubble Tuesday

MONTREAL (CNN) -- Two people were killed and one was missing Tuesday after an explosion set off by a severed gas line ripped through a homeless shelter in Montreal. At least 15 other people were injured when the facade of the three-story building collapsed.

One of the blast victims died at the scene, and another died in a hospital, both of cardiac arrest, authorities said. Hours after the explosion, a shelter volunteer was still missing in the rubble.

"We are still in shock," said Sister Nicole Fournier, one of the Catholic nuns who ran the shelter.

Catholic nun may be missing

Shelter officials said the missing person might be a nun in her 70s who is a member of the historic order of Grey Nuns. The order has provided meals and shelter for the homeless for 120 years.

The building's facade collapsed

Many of those injured were taken to nearby hospitals, where five were reported to be in serious condition with cuts, fractures and head injuries.

The explosion occurred at 12:19 p.m. just as lunch was being served at the shelter in the city's historic quarter. No one lived in the Accueil Bonneau shelter, which was used to provide meals and shelter to homeless people during the day.

Nuns and volunteers usually serve lunch to between 50 and 75 people in the brick building that was devastated by the explosion. Shelter spokesman Jacinthe Bouchard estimated that 35 people were inside the building when the blast occurred.

"It was like a building demolition. It came down vertically," said Louis Fournier, a witness.

Victims stagger from scene

After the blast, passersby and police helped pull the injured from the collapsed portion of the building. Several victims were bleeding profusely and lay immobile. Others were staggering as they were led from the building.

"It went boom and that was it," said an elderly woman as she helped a shelter volunteer walk away from the scene.

Rescue teams combing the rubble were using a trained dog to search for the missing person. Motorists in the area were asked to stop their engines to help rescuers listen for any sounds coming from the building.

Worker cut gas pipe

An elderly woman is led from the building

Police said the powerful explosion was caused by a gas leak after a worker cut into a gas conduit. An evacuation order was issued, but police said only a few people managed to leave the building before the blast.

An official at Gaz Metropolitain, the local utility, said an employee of a private contracting firm was preparing the site to install a new gas meter when the leak occurred.

"Apparently he cut a pipe. There was a gas leak," said Stephane Bertrand. "He called Gaz Metropolitain and we got here as quickly as possible."

The worker urged people inside the building to leave, Bertrand said.

After the blast, crews from the utility excavated a nearby street to reach conduits, enabling them to shut off gas to the building, which authorities said was in danger of completely collapsing.

The powerful explosion blew bricks into the street and onto cars parked outside the shelter. A male cyclist pedaling by was slightly injured.

The shelter is near the waterfront in Old Montreal, whose narrow, cobblestone streets are popular haunts for both tourists and locals. Several tour buses were parked nearby.

Montreal Mayor Pierre Bourque was in his office in city hall about 300 yards (275 meters) from the shelter when the explosion occurred.

Reuters contributed to this report.